Aspects of Hearing Aid Fitting Procedures

Sensorineural hearing loss is a common and chronic disorder that affects almost ten percent of the world population. In the Netherlands, it is also the major disorder in the working population [NCvB, 2008]. Hearing loss leads to restriction in the interaction with others and withdrawal from participation in (social) activities. Due to the size of the problem and the vast impact on the function, hearing rehabilitation is an important issue. Although hearing rehabilitation focuses on many more aspects such as learning of communication strategies and adaptation to the acoustical environment, hearing aid fitting is one of its first essential steps. Hearing aids have to amplify sound to a level above the hearing threshold to utilize the residual hearing capacity of the ear as much as possible. In the 20th century, a number of technological advances have taken place in amplification devices. These started from nonelectronic ear horns that were replaced by electronic hearing aids. Amplification was initially achieved by analogue circuits, while from the 1990s digital signal processors have entered the market. An enormously wide variety of hearing aid models has become available since [Bentler & Duve, 2000]. Aside from differences that have to do with the sound that is being produced, hearing aids can be classified with respect to type. While the technological development started with body-worn hearing aids, we nowadays distinguish behind-the-ear (BTE), in-the-ear (ITE) and hearing aids that fit partly or completely in the ear canal (CIC). These types are available in a wide variety of models, colours and sizes and are of various brands. A classical feature is the telecoil for use with induction loops. Options that are available for modern hearing aids are remote controls, infrared and fm-receivers, the use of multiple programs and water resistant housings. Last but not least, every hearing aid has its own price. It is obvious that the search for the hearing aid that is most suitable for the individual patient can be regarded as a real challenge. It is not only based on measures like speech perception but may also be determined by listening comfort, wearing comfort and functionality. This is all devised during the selection phase of a hearing aid fitting. Aside from differences in the exterior and the above-mentioned features, hearing aids can be distinguished with respect to the sound that they produce. For a long time the amount of amplification and the frequency characteristic were the main issues. Later on, electronic compression circuits were added to limit the maximum output and/or gain of the hearing aid. More recently developed features are feedback reduction, noise cancellation and the use of directional microphones. To adjust the various controls of the hearing aid in order to optimally compensate for the affected cochlea is a challenge on its own. This is done during the adjustment phase of a hearing aid fitting. Procedures for hearing aid fitting have been invented in parallel with the development of hearing aid technology.

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